Method And Device For Regenerating Hydrochloric Acid

ABSTRACT

The subject matter of the present invention is a method to extract or recover hydrochloric acid from hydrochloric acid solutions containing metal by means of pyrohydrolytic treatment, followed by absorption and/or condensation of the gaseous hydrogen chloride thus formed in order to form hydrochloric acid. According to the invention, a first partial flow of the hydrochloric acid solution containing metal undergoes pyrohydrolytic treatment and a second partial flow of the metal-containing solution is fed to the absorption column. A device for implementing the process according to the invention is also the subject of the present invention.

The subject matter of the present invention is a method to extract or recover hydrochloric acid from hydrochloric acid solutions containing metals by means of pyrohydrolytic treatment of the solution, followed by absorption and/or condensation of the gaseous hydrogen chloride thus formed in order to form hydrochloric acid.

A device for implementing the process according to the invention is also the subject of the present invention.

Hydrochloric acid containing metal arises in the metal industry, for example, during pickling of carbon steel. These solutions contain free hydrochloric acids and metals dissolved as chlorides, such as iron. The hydrochloric acid containing approximately 18% HCl and as little iron as possible is added to the final pickle section in counter-current to the strip. The spent pickling solution containing approximately 120 g/l Fe from the first pickle section is removed and regenerated.

Several methods have already been developed by which the hydrochloric acid from the spent pickling solution can be recovered.

AT395312B describes a process where the acid is recovered by spray roasting of the solution containing metal, followed by absorption and/or condensation of the gases thus formed in an aqueous absorption solution. The metal oxides generated during pyrohydrolysis are removed at the base of the spray roaster.

EP 0775760 describes a similar process for recovery of acid by means of pyrohydrolytic treatment, where the waste pickling liquor undergoes pre-concentration by evaporation before pyrohydrolysis.

The set-up of these systems is known in specialist circles or from AT395312B and EP 0775760, thus it is not described in more detail here.

The energy requirement for HCl regeneration in a conventional plant—depending on the size of the plant—is approximately 650-700 kcal per liter of waste pickling liquor fed to the plant. Normally, a gaseous fuel is used to fire the plant.

One disadvantage of the current HCl regeneration process by spray roasting is that the maximum possible metal concentration is not achieved in pre-concentration of the waste pickling liquor (Venturi loop), which means that an unnecessarily large amount of water is evaporated, causing a very high energy requirement for acid regeneration.

The invention is thus based on the task of reducing the energy requirement for acid regeneration compared to conventional spray roasting systems.

This problem is solved by means of a method to extract or recover hydrochloric acid from hydrochloric acid solutions containing metal, according to claim 1.

According to the invention, the solution containing metal is split into a first and second partial flow. The first partial flow of the metal-containing, hydrochloric acid solution undergoes pyrohydrolytic treatment. However, the second partial flow of the metal-containing, hydrochloric acid solution does not undergo pyrohydrolytic treatment, but is fed to the absorption column. This leads to an increase in the metal content of the entire pickling/regeneration system.

Thus, only a partial flow of the metal-containing, hydrochloric acid solution is fed to the pyrohydrolytic treatment and not the total quantity. As is evident from the embodiment example, the free acid content is not changed and the pickling effect is not diminished as a result.

The fuel savings potential as a result of the invention is roughly 25%, and the power consumption is also lowered in addition because the waste gas volume is reduced to the same extent.

It is preferable if the metal-containing, hydrochloric acid solution undergoes concentration by evaporation because this increases the concentration of the solution.

The gases from pyrohydrolytic treatment can be cooled by means of direct contact with the metal-containing, hydrochloric acid solution. The metal-containing solution becomes more concentrated as a result. Almost the maximum possible iron concentration can be achieved during the concentration process so less water must be evaporated.

In a special design, the first partial flow of more concentrated hydrochloric acid solution containing metal undergoes pyrohydrolytic treatment and the second partial flow of more concentrated hydrochloric acid solution containing metal is fed directly to the absorption column or can also be mixed beforehand with rinsing water.

In this embodiment of the invention, the regenerated acid has a higher trivalent iron content, which is known to enhance the pickling effect.

It is favorable if the method according to the invention is used to regenerate the hydrochloric acid from a ferrous solution. It is useful here if a regenerated hydrochloric acid with an iron content of more then 10 g/l, preferably more than 40 g/l, is obtained after the absorption column.

In the pickle, the hydrochloric acid is thus not enriched from close to 0 g/l Fe to 120 g/l Fe, as was common in the past, but from 40 g/l Fe, for example, to 160 g/l Fe or from 50 g/l Fe to 170 g/l Fe.

The method according to the invention can be used either to regenerate the hydrochloric acid from a pickling process, or also from a leaching process.

An appropriate device for extraction and/or recovery of hydrochloric acid from solutions containing metal, comprising a feed pipe for the solution, a pyrohydrolysis reactor, and at least one absorption column into which the, preferably cooled, waste gas is fed, is also the subject of the invention.

According to the invention, the feed pipe for the metal-containing solution to the pyrohydrolysis reactor has a branch piece so that only part of the metal-containing, hydrochloric acid solution can be fed to the pyrohydrolysis reactor and another partial flow of the metal-containing, hydrochloric acid solution can be fed to the absorption column.

The feed pipe for the second partial flow can either discharge directly into the absorption column or also into the feed pipe for rinsing water.

In the following, an embodiment of the invention is described on the basis of drawings. In these drawings,

FIG. 1 shows a diagram of the mass flows in a conventional pickling line with acid recovery;

FIG. 2 shows a diagram of the mass flows in a pickling line with acid recovery according to the invention;

FIG. 1 shows a diagram of a pickling line 1 with an acid recovery plant 2 (ARP) according to the state of the art.

In this pickling line, 1200 kg/h iron are dissolved at a throughput of 2 million tonnes of low-alloy steel per annum, with a pickling loss of 0.4% (as Fe). Regenerated hydrochloric acid containing 194.6 g/l free hydrochloric acid is pumped from the ARP (Acid Recovery Plant) 2 to the pickling line 1.

In the pickling line 1, the hydrochloric acid reacts essentially according to the following reaction

FeO+2 HCl=FeCl₂+H₂O

and is thus spent and converted into iron chloride.

At an iron content of approximately 121 g/l and a free acid content of 47 g/l, the pickling solution is spent and fed to the acid recovery plant (ARP) 2. The required capacity of the ARP is 10 m³/h, and the fuel gas requirement is 27896 MJ/h. For absorption and/or condensation of the gases coming from the roasting reactor, 11.11 m³ of water per hour are fed to the absorption column in the acid recovery plant 2. In addition, 1200 kg of iron are discharged from the spray roasting reactor as Fe₂O₃.

Similarly, a waste gas flow of 10.99 tonnes of water vapor leaves the ARP hourly.

FIG. 2 shows a diagram of a pickling line 1 with an acid recovery plant 2 according to the invention.

Regenerated hydrochloric acid containing 194.6 g/l of free hydrochloric acid is also pumped from the ARP (Acid Recovery Plant) 2 to the pickling line 1. Due to the fact that a part 6 of the spent pickling solution is fed to the absorption column together with rinsing water, the regenerated acid has an iron content of 39.9 g/l. In the pickling line 1, the same amount of iron is dissolved as in FIG. 1, and the hydrochloric acid is spent.

At a free acid content of 47 g/l, the pickling solution is spent and fed through the feed pipe 3 to the acid recovery plant 2 (ARP). In this case, the iron content is 161 g/l.

The feed pipe 3 to the ARP has a branch piece 4 through which a partial flow 5 of the spent pickling solution is fed to the pyrohydrolysis reactor (spray roaster) and another partial flow 6 of the spent pickling solution is fed to the rinsing water in the absorption column.

Thus, a smaller amount of spent pickling solution compared to FIG. 1 is fed to the pyrohydrolysis in order to extract the same amount of iron (1200 kg/h). In this case, only 7.52 m³/h are fed to the pyrohydrolysis stage of the ARP, and the remaining 2.48 m³/h are fed to the absorption column together with the water. Thus, the fuel gas requirement is only 21319 MJ/h.

As a result, 23.6% of the fuel gas is saved.

In addition, electrical energy is saved as well, mainly for the exhaust air fan, because the volume of process gas drops by the same proportion. Now only 8.2 tonnes of water vapor per hour leave the ARP with the waste gas.

As both the regenerated and the spent acid each have approximately the same free HCl content in the two embodiment examples, the process according to the invention does not have any negative influence on the pickling effect.

In another variant of the invention, the concentration of the spent acid is increased first of all in a cooling unit for the hot waste gases, e.g. in a Venturi loop. The concentrated, metal-containing solution (concentrate) is then split into a first and second partial flow. The first partial flow is fed to the pyrohydrolysis reactor and the second partial flow from the concentrated waste pickling liquor is fed to the absorber or to the rinsing water going to the absorber.

The results of some calculations are summarized in Table 1:

TABLE 1 Roaster Spec. energy Fe (g/l) in Fe (g/l) in feed consumption Fuel waste picking regenerated Fe (g/l) quantity (kcal/l waste saving liquor acid in concentrate (m³/h) pickling liquor) (%) 1 Conventional 120 0 202 6.64 666 — process (FIG. 1) 2 Waste pickling 160 40 270 4.99 509 −23.6 liquor to absorber 3 Waste pickling 170 50 284 4.68 479 −28.0 liquor to absorber 4 Concentrate to 160 40 237 5.65 570 −14.4 absorber 5 Concentrate to 170 50 245 5.45 550 −17.4 absorber

The comparison in Table 1 shows that the energy-saving potential is greater if the waste pickling liquor is fed to the absorber rather than its concentrate. The metal content in the regenerated acid is controlled via the amount of metal-containing solution mixed into the rinsing water for feeding to the absorber. Rinsing water consumption diminishes accordingly.

In HCl regeneration for 5.3 m³/h waste pickling liquor (for an annual pickling capacity of approximately 1 million tonnes), this results in savings of >200 k

/year based on European energy prices.

The investment costs for a new plant are also lower.

In existing plants, the invention can be used to achieve a significant increase in performance. 

1. Method to extract or recover hydrochloric acid from hydrochloric acid solutions containing metals by means of spray roasting followed by absorption and/or condensation of the gaseous hydrogen chloride thus formed in order to form hydrochloric acid, where a first partial flow of the metal-containing, hydrochloric acid solution undergoes pyrohydrolytic treatment and a second partial flow of hydrochloric acid solution containing metal is not subjected to the pyrohydrolytic treatment, but is fed directly to an absorption column, wherein the improvement comprises that the metal-containing hydrochloric acid solution undergoes concentration by evaporation before pyrohydrolytic treatment.
 2. Method according to claim 1, wherein gases from pyrohydrolytic treatment are cooled by direct contact with the metal-containing, hydrochloric acid solution, where the concentration of metal-containing solution is increased and where the first partial flow of more concentrated hydrochloric acid solution containing metal undergoes pyrohydrolytic treatment after this and the second partial flow of more concentrated hydrochloric acid solution containing metal is fed to the absorption column.
 3. Method according to claim 1, wherein the second partial flow of hydrochloric acid solution containing metal is mixed with rinsing water before being fed to the absorption column.
 4. Method according to claim 1, wherein the hydrochloric acid is extracted from a ferrous solution.
 5. Method according to claim 4, wherein a regenerated hydrochloric acid with an iron content of more then 10 g/l is extracted after the partial flow of hydrochloric acid solution containing metal passes through the absorption column.
 6. Method according to claim 1, wherein the hydrochloric acid solution containing metal originates from a pickling process and the regenerated hydrochloric acid containing metal is recycled to the pickling process.
 7. Method according to claim 1, wherein the hydrochloric acid solution containing metal originates from a leaching process and regenerated hydrochloric acid containing metal is recycled to the leaching process.
 8. Device to extract or recover hydrochloric acid from a solution containing metal, comprising a first feed line (3) for the solution containing metal leading to an acid recovery plant (2), where the acid recovery plant (2) has a spray roasting reactor for pyrohydrolytic treatment of the solution containing metal and at least one absorption column for absorption and/or condensation of gases manating from the spray roasting reactor, where the first feed line (3) or a second feed line to the spray roasting reactor has a branch piece (4) such that only a first partial flow (5) of the solution containing metal can be fed to the spray roasting reactor and where a second partial flow (6) of the solution containing metal can be fed to the at least one absorption column, wherein the improvement comprises that the solution containing metal undergoes concentration by evaporation before a pyrohydrolytic treatment.
 9. Device according to claim 8, wherein the feed line (3) discharges into a cooling device which is disposed in an exhaust gas pipe between the spray roasting reactor and the at least one absorption column such that, waste gases from the spray roasting reactor can be brought into contact with the solution containing metal for cooling purposes, where the solution containing metal must pass through the cooling unit before entering the spry roasting reactor, and where the branch piece (4) for the first partial flow (5) and second partial flow (6) is disposed downstream of the cooling unit.
 10. System for extracting or recovering hydrochloric acid from a solution containing metal, comprising: a pickling stage (1); an acid recovery plant(2); at least one absorption column; a first feed line (3) connected to the pickling stage (1); a branch piece (4)connected to the first feed line (3); a first flow path (5)connected to both the branch piece (4) and the acid recovery plant (2); and a second flow path (6)connected to both the branch piece (4) and the at least one absorption column; wherein the acid recovery plant further comprises a pyrohydrolysis reactor and further wherein the solution containing metal undergoes concentration by evaporation prior to entering the pyrohydrolysis reactor.
 11. Method according to claim 1, wherein the first partial flow of the metal-containing, hydrochloric acid solution undergoes concentration by evaporation.
 12. Method according to claim 1, wherein both the first partial flow of the metal-containing, hydrochloric acid solution undergoes pyrohydrolytic treatment and the second partial flow of hydrochloric acid solution containing metal have undergone concentration by evaporation.
 13. Device according to claim 8, wherein the first partial flow (5) of the solution containing metal undergoes concentration by evaporation.
 14. Device according to claim 8, wherein the first partial flow (5) of the solution containing metal and the second partial flow (6) of the solution containing metal have undergone concentration by evaporation.
 15. The system of claim 10, wherein the solution containing metal undergoes concentration by evaporation in the acid recovery plant (2).
 16. The system of claim 10, wherein the solution containing metal undergoes concentration by evaporation in the first flow path (5).
 17. The system of claim 10, wherein the solution containing metal undergoes concentration by evaporation in the first feed line (3). 